Air suspension system in microwave drying



June 17, 1969 SCHREIBER ET 3,449,835

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AIR SUSPENSION SYSTEM IN MICROWAVE DRYI NG Filed Oct. 25. 1967 Sheet of s 1 1 [El- E INVENTORS 52%; M W 1541f? JERO E IN ERK W STEPHANSEN A TOKNEYS United States Patent 3,449,836 AIR SUSPENSION SYSTEM IN MICROWAVE DRYING Henry Schreiber, Port Washington, N.Y., and David J. Goerz, Jr., Menlo Park, Jerome W. Hankin, Morega, and Erik W. Stephansen, San Francisco, 'Calif., .assignors, by mesne assignments, to Bechtel International Corporation, San Francisco, Calif., a corporation of Delaware Filed Oct. 25, 1967, Ser. No. 677,914 Int. Cl. H05b 9/00; F26b 3/28 US. Cl. 34--1 7 Claims ABSTRACT OF THE DISCLOSURE A drying device for a web wherein the solvent to be removed from the web is subject to microwave heating and high speed gas jets by passing the web through a split waveguide whereon the otherwise open portions of the waveguide are covered by an electrically non-conducting low loss-tangent material to provide an air tight seal over the open portion of the waveguide for its entire length to prevent moisture and dust from collecting in the waveguide and disturbing the electrical properties. Web suspension is provided by the high speed jets and may be provided by passing gas through the waveguide cover through small holes or slots.

A dryer for paper or other materials containing solvent or moisture wherein microwave energy is combined with high velocity gas impingement by passing the material between the two halves of a split waveguide. The two halves are substantially sealed by a dielectric low losstangent material, preventing entry of moisture or solvent or any other foreign materials into the waveguide and providing an air suspension system for the web.

Description of the prior art Heretofore microwave energy has been employed together with high speed gas or air jets for the drying of paper and similar materials, the microwave energy supplying heat to vaporize the solvent while the high speed gas jets break down the boundary layer and carry away the released solvents. In such a system there is always the possibility of moisture or lint entering the waveguide itself and impairing its efficiency. In accordance with the present invention, a split waveguide is used wherein the two portions of the Waveguide are substantially sealed by the use of a dielectric low loss-tangent material, preventing entry of air or moisture into the guide. Thus, lint and other web borne solids are prevented from entering or accumulating in the meanderline waveguide or other microwave applicator device. Further, the flexible web is supported under normal operating tension within the drying apparatus by the action of the gas applied to its two surfaces, the action of the gas applied to the web being such that the web will not contact any part of the drying apparatus while passing through it. Thus, moisture or solvent released from the web by absorption of microwave energy is kept from condensing on any part of the waveguide and associated gas handling apparatus. Since the two halves of the waveguide are substantially closed, threading can easily be accomplished with the applicator either open or closed without the danger of the web getting caught in a waveguide or other microwave power application device, or in the associated gas handling equipment.

Summary of the invention The preferred embodiment consists of a fully split rneanderline microwave applicator provided with a high 3,449,836 Patented June 17, 1969 velocity gas supply and exhaust system. In this embodiment, the open portion of the split meanderline waveguide is covered by an electrically non-conducting, low losstangent material in such a manner as to provide an air tight seal over the open portion of the waveguide for its entire length. The cover over the open waveguide can be provided with holes distributed along its length to permit the passage of gas from within the pressurized waveguide onto the web moving past it. Spacing of the holes in the waveguide cover will be such that a continuous gas pressure cushion is formed between the moving web and the waveguide cover on each side of the web. Gas for this purpose is admitted to the back of the waveguide through holes spaced at /a waveguide wave length intervals. If the web moves closed to one covered half waveguide, thereby moving further from the one opposite, the gas pressure in the gas cushion adjacent to the first half waveguide will increase, while the gas pressure in the cushion adjacent to the opposite covered half waveguide will decrease, resulting in a net pneumatic force being exerted on the Web in such a direction as to center it between the two covered half guides.

The impingement jets are arranged in such a manner that the velocity vector exerts a net fluid force on the web in its direction of travel, while retaining a velocity component normal to the web of suflicient magnitude to disrupt the solvent boundary layer adjacent to its surface.

In a preferred embodiment of the invention, the leading edges of all parts of the drying apparatus being passed by the web are curved in the direction of motion of the web to develop aerodynamic forces which provide better flotation stability to the web and to prevent the web from catching while threading.

Preferably the return gas plenums are covered at intervals of between and 3" by strips of an electrically non-conductive, low loss-tangent material arranged longitudinally in the direction of motion of the web. These strips, whose spacing is preferentially staggered from one return gas plenum to the next, serve to keep the web from entering the return plenums during threading or in case of a web break.

In another embodiment of the above invention, all parts of the apparatus are as above described, except that the waveguide is not provided with gas supply holes and its cover is not provided with web balancing holes. The waveguide is thus hermetically sealed.

In still another embodiment of the invention, the waveguide is completely filled With a non-conductive, low loss-tangent material such that each half waveguide has a smooth surface adjacent to the web passing between the two half waveguides.

In still another embodiment of the invention, the apparatus is identical with that described above, except that A to A holes are drilled from the back of the Waveguide, through the nonconductive, low loss-tangent filling within the waveguide, these holes being spaced at half waveguide wave length intervals along the length of waveguide addressing the web. These holes serve to convey gas to the proximity of the web for suspension purposes as described in the preferred embodiment.

Brief description of the drawings FIGURE 1 is a perspective, fragmentary view of a dryer embodying the present invention.

FIGURE 2 is a side view in section of a dryer embodying present invention.

FIGURE 3 is an exploded view of a dryer embodying the present invention, showing the two halves of the waveguide and associated air flow equipment separated for clarity and some of the parts cut-away. The overall air flow and microwave energy flow through the device is illustrated schematically.

FIGURE 4 is a cross section through a device embodying the present invention.

FIGURE 5 is a detailed view of an alternate waveguide construction.

FIGURE 6 is a view on the line 66 of FIGURE 5.

Description of the preferred embodiment The present invention is an improvement on the device shown in the application filed by three of us on or about Oct. 10, 1967, Ser. No. 674,293. Some of the parts which are common to the two applications are not described in detail herein.

Referring now to the drawings by reference characters there is shown a web 10 being drawn through the dryer by means well known to those skilled in the art and not illustrated. The web can move in the direction shown by the arrow or the opposite direction. Gas under pressure is supplied by a fan 12 through ducts 14 to the inlet plenum 16. A heater and suitable filters may be included in the air circuit. Air is exhausted for the dryer through the outlet plenum 20 through line 22. Normally a return line is provided with suitable dampers so that a large portion of the air can be recirculated to conserve heat. A source of microwave energy 36 is provided which feeds the waveguide through line 38. The generator may be any suitable RF generator operating at a frequency of from 300 to 30,000 mH.z, such as on the ISM allocated frequencies of 915, 2,450, 5,800 and 22,125 mHz. In one practical embodiment, this is a klystron and its associated electronic equipment operating at 2,450 mHz.

The energy is fed into the waveguide, generally designated 40, which is split into two matching halves. Power is introduced to the structure through a split microwave T with the opposite half of the waveguide being provided with an impedance matching stub 44 tuned with a piston 46 which is provided with an adjusting screw 48. At the outlet end a dummy load 42 is suitably provided which can take the form of a water filled tube 50 set at an angle as shown. The opposite terminal half of the waveguide may be provided with a matching stub 52.

The waveguide consists of two complementary halves so that it is only neccessary to describe one half in detail. The waveguide includes a back wall 54 and side walls 56 and '58 which are either of a conductive material or of a non-conductive material with a layer of conductive ma-* terial coating the interior surfaces. The side wall 58 can be made as a continuation of the wall 60 which divides the inlet and outlet plenums 16 and '20. In accordance with the present invention, the two electrically open halves of the waveguide are mechanically sealed with a plate, generally designated 55, which is made of a nonconducting, low loss-tangent material. Typical suitable materials are Teflon (polytetrafluoroethylene) and polypropylene although it will be recognized by those skilled in the art that any material which has the necessary electrical and mechanical properties may be employed. Thus, the web 10 passes between the two plates 55 and is suspended between these plates by means of the gas jets hereinafter described.

The plates 55 have a series of openings 57 set therein which are high speed gas jets connected by means of the holes 62 to the inlet plenum 16. Holes 62 connecting the plenum 16 to the jet outlets '57 are preferably set at an angle, as is best seen in FIGURES 2 and 5, the function of the angle being to tend to propel the paper through the machine, thus simplifying threading and also directing the air flow away from the waveguides and into the air outlet plenum 20. In order to prevent the flow of air into the waveguide, the waveguide is preferably pressurized by applying air or gas from the inlet plenum '16 through the openings 68 into the waveguide. In plate 55, openings 59 are provided so that gas flows out of the waveguide onto the web, also helping to dry and center the web. Dielectric material 61A is employed to seal the ends of the waveguide.

It will be noticed that the holes 68 are spaced at intervals of about one-half waveguide wave length along the length of the waveguide to avoid electrical interference with the waveguide function. On the other hand, the spacing of the holes 59, being in the insulating material, is not critical.

In order to provide mechanical integrity for the structure and to prevent the web from being caught in the outlet plenum, straps 61 are set at intervals across the outlet plenum. These can be formed in the sheet 55 or the sheet and the straps can be made of separate pices of material.

The waveguide heretofore described has been hollow and this configuration is normally preferred. However, the waveguides may be formed with a filling of a low loss-tangent plastic material such as polystyrene foam as is shown at in FIGURE 5. Since it is still desirable to provide some out flow of air from the waveguides, holes 63 can be provided between the waveguide outlets 59 and the inlets 68. These holes 63 are formed in the plastic filling 65 at angles as is shown since, as has been mentioned before, the holes 68 should not recur at intervals of less than one-half waveguide wave length while the holes 59 occur at close intervals along the length of the waveguide.

Preferably all of the leading surfaces that the web may encounter within the machine are cut away as at points 69 and 71 which greatly lessens the chance of the web catching on any part of the machine and aids in the threading of the machine.

A three-pass waveguide system is shown but any number of pases might be employed.

Although certain configurations of the dryer have been shown and described, it will be recognized that the invention is one of broad applicability and that these specific embodiments are for purposes of illustration only.

We claim:

1. An apparatus for drying a web comprising in combination:

(a) means for moving a web through said apparatus,

(b) a conductor of electromagnetic energy adjacent to the web which produces an electromagnetic field in said web, said conductor comprising a split waveguide having two portions with one portion on each side of said web,

(0) high speed gas jets in said apparatus, said jets impinging on both sides of said web and suspending the web there between and including means for supplying gas to said jets and (d) a low loss-tangent material substantially enclosing the otherwise open portions of said waveguide.

2. The structure of claim 1 wherein the high speed jets are arranged adjacent to said waveguide whereby the web is subjected successively to microwave energy and high speed jets of gas.

3. The structure of claim 1 wherein the low loss-tangent material has openings therein with means to supply gas under pressure to the interior of said waveguide whereby gas passes outwardly through said openings in said low loss-tangent material.

4. The structure of claim 1 wherein the leading edges of all parts of the drying apparatus which are passed by the web are curved in the direction of motion of the web.

5. The structure of claim 1 wherein return gas plenums are provided at intervals along the web and wherein said plenums are covered by spaced strips of an electrically non-conductive low loss-tangent material located longitudinal to the direction of motion of the web.

6. The structure of claim 1 wherein the interior of the waveguide is filled with a foam of a non-conductive low loss-tangent plastic material.

7. The structure of claim 6 wherein holes are provided in the back of the waveguide and through the non-conductive low loss-tangent filling within the waveguide, terminating in holes along the length of the waveguide for FOREIGN PATENTS suspending the web between the waveguide segments. 1,050,493 12/1966 Great Britain References Cited UNITED STATES PATENTS 5 US. Cl. X.R. 3,277,580 10/1966 Tooby 34-1 219-10.55, 10.61

KENNETH W. SPRAGUE, Primary Examiner. 

